Electrical Circuits (1 Viewer)

[davidbarnes]

How does electrical energy flow around a circuit. The negative atoms are tring to push their way around a circuit to reach the positive terminal, hence generating a charge?

Can electrical energy only flow from positive to negative, or can it also flow from negative to positive?

 

[twilight1412]

this is how it works

until you reach uni the only thing you are concerned about is the conventional current which is positive to negative

yet we know this is not true because you get electrocuted when youo stick your finger into the negative terminal of a dc power pack (dont try at home)

dc current is one way therefore it is called 'direct current'

but ac current is slightly different
the current moves back and forth in both directions at the rate of about 50 direction changes per second

it is therefore called alternating current as the direction of the current alternates

 

[alcalder]

Conventional Current flow is from the positive terminal to the negative terminal in a circuit.

However, current flow is actually due to electrons from the atoms flowing from the negative terminal to the positive terminal (in the opposite direction to conventional current. An unfortunate historical oops.)

You could also say that there is a "flow" of holes in the direction of conventional current. This is not really a flow or movement of anything, only the electrons are moving but picture this.

Imagine your physics class forms a circle. Every person, but one, has a ball. Now pass the ball to the person without a ball. Keep doing that all the way around the circle. You will see the balls (the electrons) are flowing in one direction but the hole left by the moving electron moves in the opposite direction.

Does that help?

 

[twilight1412]

umm alcalder arent you goin a bit too far? your starting to talk about semicondutors ><

 

[davidbarnes]

C
You could also say that there is a "flow" of holes in the direction of conventional current. This is not really a flow or movement of anything, only the electrons are moving but picture this.

Imagine your physics class forms a circle. Every person, but one, has a ball. Now pass the ball to the person without a ball. Keep doing that all the way around the circle. You will see the balls (the electrons) are flowing in one direction but the hole left by the moving electron moves in the opposite direction.

Does that help?

Thats helpful thanks. I acrtually understand it.

 

[alcalder]

In some ways it does encompass semi-conductors (and you now have aheads up on that) but it can also be said for electric circuits and metallic conductors.

Those "free electrons" are not exactly free, they are valence electrons which are just easily knocked into the conduction band with the addition of a small amount of energy. So there will be holes.

 

[Riviet]

umm alcalder arent you goin a bit too far? your starting to talk about semicondutors ><

You will be learning more about semiconductors in the module, Ideas to Implementation.

 

[alcalder]

Indeed you will, but never lose sight of the fact that Physics is a very integrated subject. You will use ideas and concepts across multiple topics. This is important when you decide on which equation to use. You may be studying ideas to implementation but may need to use an equation from Electrical Circuits (or even the Prelim course). Beware!

 

[Forbidden.]

How does electrical energy flow around a circuit. The negative atoms are tring to push their way around a circuit to reach the positive terminal, hence generating a charge?

Can electrical energy only flow from positive to negative, or can it also flow from negative to positive?

Direct Current (e.g batteries):

+ and - : Yes
Direction: - to + only.


Alternative Current (e.g powerpoints):
+ and - : No
Direction: Back and forth.

You can take 200mA of DC current but 200ma AC current will kill you.

 

[Forbidden.]

Indeed you will, but never lose sight of the fact that Physics is a very integrated subject. You will use ideas and concepts across multiple topics. This is important when you decide on which equation to use. You may be studying ideas to implementation but may need to use an equation from Electrical Circuits (or even the Prelim course). Beware!

You mean:

Prove that P=V²/R and P=I²R
if P=VI and V=IR

Proof:

P=VI

But V=IR

P=(IR)I
P=IxIxR

.=I²R

But I=V/R

P=(V/R)²/R
P=V²/R²/R

.=V²/R

 

[davidbarnes]

Ok, got a test on this tomorrow, so would appreicate help on the below, which I don't get.

In simple terms how, do generators aand motors work? I'm not meaning current + magnet field, though.

Can current frlow from neagtive to positive. I'm not meaning negatively charged atoms flowing either.

I've just got a mental blank on this question, and can't work out the formula.
An iron has resistance of 75 Ohms. What current flows when the heating element is 240 V?
So the normal formula would be R = V OVER I, would I use I = R over V?

Does anyone know of a good site/image on the net showing a circuit with resistances in paralell for practical use, e.g, lighting a home, etc.

What affect would a: direct current, or b: alternating current have on a loudspeaker?

Thanks for any helpful pointers.

 

[davidbarnes]

What wil be the direction of 4 compass needles palced on the card around the wire?

 

[alcalder]

In simple terms how, do generators aand motors work? I'm not meaning current + magnet field, though.

A generator is making electricity, so the electrons are being given energy and so they flow from the positive to the negative side of the generator. (Hence current flows from negative to positive because it is essentially INSIDE the battery or power source.)

A motor is using electricity, so the electrons are using energy and flow from the negative to the postive side of the motor.

An iron has resistance of 75 Ohms. What current flows when the heating element is 240 V?
So the normal formula would be R = V OVER I, would I use I = R over V?

V=IR Thus
R=V/I
I=V/R

All the same equation just moved around a bit.

So I = 240/75 OR

240=I x 75 OR

75 = 240/I

will all yield the same result.

Does anyone know of a good site/image on the net showing a circuit with resistances in paralell for practical use, e.g, lighting a home, etc.

As for practical circuits for this, I'd pull out your user manuals for stereos and such around the house. Even the wiring diagram for your car. There is likely to be some practical parallel circuits in there.

What affect would a: direct current, or b: alternating current have on a loudspeaker?

In motors and generators you will learn that only a changing electric current can induce a magnetic field. A loud speaker is basically a paper cone on a spring which dances around when a changing current induces a magnetic field which in turn induces a changing current in the loud speaker's spring. An AC current would make the loud speaker hum. A DC current would give a noise as it powers up and when it powers down but nothing in between.

 

[alcalder]

What wil be the direction of 4 compass needles palced on the card around the wire?

If the wire held a current that was VERY VERY BIG and the wire was held vertical, then you would see the four compass needles circle around the wire. Using the right hand grip rule, hold the wire with your thumb in direction of the current (well, don't actually hold a wire with a VERY VERY BIG current in it, but imagine you are ).

A magnetic field runs from north to south and the "north" end of a compass is a "north" pole. SO, if the current is running up the wire, the magnetic field runs anti-clockwise so the compass needles will point in an anti-clockwise direction.

(Remember, although a compass points to the Earth's North Pole - capital letters - the Earth's North Pole is actually a south magnetic pole.)

NOW, in a physics classroom experiment, the amount of current in your wire will not be VERY VERY BIG and so you will have the Earth's magnetic field interacting with your compass. THe resultant deflection of the compass needle will be a vector addition of the Earth's magnetic field and the magnetic field due to the current. SO, the compass needle will only deflect by a small amount.

 

[davidbarnes]

Thanks for all that Alcalder. I think I done not bad in the test, and hopefully got 85/90% correct. Know I stuffed up one question on teh charge of electrons.

As for how a motor and generator works, I meant like the brushes in a motor, and how they touch the turning coil, and the cuts in a communtator, etc.

 

[alcalder]

As for how a motor and generator works, I meant like the brushes in a motor, and how they touch the turning coil, and the cuts in a communtator, etc.

In that case, I'm not quite sure what it is you want to know.

The "brushes" brush against the turning ring (commutator) and therefore complete the circuit and bring the current to the motor coil (or away from the generator coil).

It is the coil's momentum that gets the brushes over the Split ring commutator in the case of a DC motor.

Here is a neat demo:

http://www.edumedia.fr/m193-p2_l2-circuits-and-devices.html

Does that help?

 

[clonestar]

How does electrical energy flow around a circuit. The negative atoms are tring to push their way around a circuit to reach the positive terminal, hence generating a charge?

Can electrical energy only flow from positive to negative, or can it also flow from negative to positive?

Electrical energy flows as a result of the atoms in the metal being a conductor. This creates a band structure and hence these free energy levels are now able to accept the moving negative charge called electrons around the wire.

Electrical energy is by definiton today only electron flow. This is the only way electricity flows. However, previously conventional current i.e. + charge flow was incorrectly used to explain theories and ideas relating to electricty.

Hope this clarifies your questions.

 

[davidbarnes]

Thanks for the help guys.

That link you poosted Alcader is very good.